1. Field of the Invention
This invention relates to a technology for detecting misfire occurring during operation of a gasoline or other spark-ignition internal combustion engine, and more particularly, to an ignition coil unit with a capacitor for detecting ignition voltage to be used in such misfire detection.
2. Description of the Prior Art
As is well known, in gasoline and other types of internal combustion engines, a high voltage produced by an ignition coil is distributed to spark plugs at the engine cylinders by an ignition distributor. At each cylinder, the resulting electric discharge between the spark plug electrodes produces a spark which ignites an air-fuel mixture that has been drawn into the cylinder and compressed at the proper time, causing the mixture to burn explosively. In the course of this ignition-combustion process, the mixture may for some reason occasionally fail to burn properly. This is referred to as a misfire. Misfires can result from causes in either the fuel supply system or the ignition system. Misfires caused by problems in the fuel supply system are the result of an overly lean or overly rich air-fuel mixture. At that instance, a spark is produced between the spark plug electrodes but the air-fuel mixture does not ignite. Misfires caused by problems in the ignition systems are the result of spark plug electrodes fouling or ignition circuit malfunctions, either of which may prevent normal spark discharge.
The occurrence of misfire in the course of engine operation not only degrades engine performance but may also degrade fuel efficiency. It may further cause after-firing of unburned gases in the exhaust system, which can affect the exhaust emission control system and have other adverse effects. Moreover, since the occurrence of even a single misfire indicates a misadjustment or malfunction in the fuel supply system or ignition system, prompt elimination of the problem is essential. Because of this, there is a strong need for development of a detector for detecting misfires as soon as they occur.
One type of misfire detector that has been proposed is the mis-spark detector described in Japanese Laid-open Patent Publication No. 52(1977)-118135. As shown in FIG. 21, the detector includes a conductor 51 wrapped around a portion of a high-voltage (high tension) cable 50 of the engine ignition system so as to constitute a detective capacitor 52 (a type of capacitance probe) in which the insulation cladding 50A of the high-voltage cable 50 serves as the dielectric. A voltage divider capacitor 53 is connected between the capacitor 52 and the ground so that the ignition voltage (secondary voltage of the ignition coil) applied to the conductive core 50B of the high-voltage cable 50 induces a voltage across the terminals of the capacitor 52 owing to its static capacitance. The induced voltage is statically divided by the capacitor 52 and the capacitor 53, and the voltage across the terminals of the capacitor 53 (the divided voltage) is forwarded as a detection voltage to an electronic circuit 54 for processing and discrimination. The electronic circuit 54 discriminates the occurrence of misfires from the difference between the wave form of the ignition voltage at the time of normal spark discharge and that at the time of no spark discharge (mis-sparking). Among the different types of misfires, the detector thus detects misfires that occur when no spark discharge is produced owing to a problem in the ignition system.
Another detector for detecting misfire in internal combustion engines is disclosed in the present assignee's Japanese Laid-Open Patent Publication No. 5(1993)-65868. In this detector, the ignition voltage is similarly detected from a high-voltage cable or the like of the ignition system using static voltage division, and misfiring owing to causes in the fuel supply system is detected based on the fact that, even when spark discharge occurs, the wave form of the ignition voltage differs between the case where normal combustion occurs and the case where it does not.
In conventional misfire detectors, a "capacitance probe" is used as a means for detecting ignition voltage. This probe is constituted by wrapping a sheet or ribbon of conductor around the high-voltage cable of the ignition system, so as to form a detective capacitor between the conductor and the core of the high-voltage cable using insulation cladding of the high-voltage cable as the dielectric. However, the capacitance probe constituted in this manner has a major drawback that derives from the nature of the high-voltage cable of the ignition system. Because of its flexibility and elasticity, the high-voltage cable is highly susceptible to vibration. It is also easily affected by changing ambient humidity, wetting by leaking water, and fouling with oil, grime and the like. When a capacitor for use in detection is formed by wrapping a conductor ribbon around the cable, the static capacitance of the capacitor is apt to be changed from its proper value by a shifting of the conductor caused by vibration, as well as by changing humidity, wetting with water, and fouling with oil, grime and the like. Although some change in static capacitance can be tolerated if the capacitor is to be used only for checking the ignition voltage, even slight changes have to be avoided when it is used for misfire detection, because such detection generally requires accurate detection not only of the ignition voltage but also of the ignition voltage wave form. The capacitance changes to which the prior art capacitance probe is susceptible, may alter the detected voltage wave form, and thereby make it impossible to detect misfire with high reliability.
In addition, the insulation cladding of the high-voltage cable is generally formed of synthetic rubber, a material that is readily degraded when exposed to heat and/or fouled with oil and grime. This degradation of the insulation cladding after the detection capacitor has been formed by winding the conductor around the high-voltage cable not only produces a progressive change in the static capacitance of the capacitor over time, but may also reduce the electric insulation property of the cladding to the point that the high ignition voltage can leak to the conductor wrapped around it. When this happens, the high leak voltage is apt to be conducted to the electronic circuitry of the misfire detector, which it can damage or cause to malfunction.
In actual practice, moreover, the flexibility and elasticity of the high-voltage cable make the work of attaching the conductor for forming the capacitor on the insulation cladding of the high-voltage cable and securing it thereon difficult and troublesome. Maintenance of the so-formed capacitor is also troublesome.
In order to solve these problems, the present assignee proposed an ignition coil unit in an application filed in the United States on Apr. 26, 1993 having Ser. No. 08/051,668, in which the conductor, which constitutes the capacitor for detecting ignition voltage, is embedded in the insulative resin that fills the ignition coil unit case. More specifically, it is an ignition coil unit having a metallic core, a primary coil and a secondary coil wound around the core, a first connector section for supplying a primary current to the primary coil, and a second connector section for extracting a secondary current carrying a high voltage to the exterior; all of which are molded integrally in the unit case by an insulative resin material. The detective conductor is located around a conductor constituting the second connector section, maintaining a predetermined therefrom and so forming the detective capacitor therewith.
With this arrangement, since the detective conductor, constituting a part of the detective capacitor for detecting the ignition voltage, is provided in the structurally sturdy ignition coil unit case, differently from the conventional arrangement in which the detective conductor is provided on a high-voltage cable of the ignition system, there is no danger of positional shifting of the detective conductor due to mechanical vibration, or of it being affected by ambient humidity, wetting by leaking water, or fouling with oil or grime. The detective capacitor is thus kept free from the change in its static capacitance, and the wave form of the ignition voltage can therefore be accurately detected at all times. Further, the material to be used as the insulator (dielectric) intervening between the detective conductor and the secondary connector section's inner conductor is not limited to materials such as rubber or the like used in the conventionally arrangement, which are highly susceptible to degrading. Instead, it becomes possible to use the resin used in the ignition coil unit fabrication, or a ceramic; either of which has excellent structural durability. It therefore becomes possible to prevent leaking of the aforesaid high voltage, which would otherwise cause damage to or a malfunction of the misfire detector. Furthermore, since the conductor is disposed integrally in the coil unit case at the time of molding it, it can eliminate the tedious attaching or assembling work and the troublesome maintenance previously required.
With the arrangement proposed in the earlier application, it becomes possible to solve the problems experienced in the conventional arrangement in which the detective conductor is provided on the high voltage cable.
Aside from the above, when using the capacitance probe technique, a detected voltage and hence the detection accuracy will increase with increasing capacitance of the detective capacitor. In the arrangement proposed in the earlier application, however, it is difficult in practice to increase the capacitance of the detective capacitor, and hence, it may not be necessarily sufficient for accurate detection.
More specifically, the capacitance is determined by the area of and the distance between opposed electrodes; i.e., the capacitance and hence the detection accuracy increases with decreasing distance or increasing area. In the arrangement proposed in the earlier application, however, since the detective conductor is located around the secondary connector section's conductor at a fixed distance so as to form the detective capacitor therewith, in order to increase the capacitance and enhance the detection accuracy, it becomes necessary to decrease the distance between the detective conductor and the secondary connector section's conductor, or to increase the area of the conductors. However, it is not easy to decrease the distance between the conductors due to problems such as corona discharge, current leakage, or possible deformation of the detective conductor which could occur at the time of injecting a resin in the coil unit case. As regards the area, on the other hand, it is determined by the outer diameter of the secondary connector section's conductor, and such diameter is determined by that of connectors widely used and easily available in the market. It is possible, needless to say, to choose a particular connector having greater outside diameter. However, the choice will not only invite the increase in cost of the connector itself, but also require the change in design or configuration in the other parts, resulting further increases in cost.